City and Guilds 2395 Exam Questions 1


A garage has just been built adjacent to the house of Mr P. A. Broon of 7a Glebe Street, Auchenshuggle, AUC XYZ. The garage is supplied from the consumer unit in the house by a 4 mm2 two core pvc/swa cable protected by a 32A BS EN 60898 Type B mcb. The consumer unit is in the hall cupboard. The supply is taken to a 4way distribution board in the garage. There is a BS 60947-3 incoming switch.

There are two circuits in the garage: 

  • A lighting circuit supplies four 70W fluorescent lamps controlled by two way switches at the side door and the main door respectively. It is run in 1.5/1 mm2 pvc twin and earth and protected by a 6A BS EN 60898 Type B mcb.
  • A radial socket circuit supplies two double socket outlets. It is run in 2.5/1.5 mm2 pvc twin and earth and protected by a 16 A, BS EN 61009 RCBO, Type B with a 30 mA rated residual operating current.

The wiring in the garage is surface run, clipped direct to the wooden framework of the garage. 

The house supply is 230 V, 50 Hz, single phase TN-S. Ze is 0.2Ω and IPFC is 1.5 kA. 

The earthing conductor is 10 mm2 copper and there are 6mm2 copper main equipotential 
bonding conductors to the gas, water and central heating systems. The main fuse is a 100A 
BS 1361 fuse.

The house installation was recently given a periodic inspection and test and 
found to be in good condition.

No observations or recommendations were made. 

The mcb’s and RCBO all have a breaking capacity of 6 kA.

The new installation has been inspected and found to comply with BS 7671.

The following test results have been obtained: 

Cable feeding the garage:

  • R1+R2= 0.19Ω
  • Insulation resistance >20MΩ between line and neutral
  • Insulation resistance >20MΩ between live conductors and earth

At garage distribution board :

  • Ze = 0.4Ω
  • Prospective short-circuit current Isc = 790 A
  • Prospective earth fault current    Ief =590 A 

Lighting circuit:

  • Rl +R2 = 0.31Ω
  • Insulation Resistance > 20 MΩ between line and neutral
  • Insulation Resistance > 20 MΩ between live conductors and earth
  • Zs = 0.67Ω

Socket circuit :

  • R1 +R2 = 0.255Ω 
  • Insulation Resistance >20 M Ω between line and neutral
  • Insulation Resistance > 20 MΩ  between live conductors and earth

RCBO:

  • 15mA test current: 0°=No trip, 180°=No trip
  • 30mA test current 0°=45ms, 180°=35ms
  • 150mA test current  0°=7ms, 180°=17ms

Maximum allowable values of Zs for BS EN 60898 Type B mcb’s are as follows:

  • 6A rating = Zs 7.76 Ω
  • 16A rating = Zs 2.87Ω
  • 32A rating = Zs 1.44Ω

Serial numbers of the test instruments are as follows:– 

  • IR/Continuity tester = 1234
  • EFLI/PSCC tester = 56789
  • RCD tester = AOK1

Questions:

  1. Confirm that the measured values of Zs are satisfactory using the Rule of Thumb’. Show all calculations.
  2. When would the insulation resistance test on the lighting circuit have to be done to ensure that all the wiring of the circuit was tested?
  3. Explain why a RCBO has been used to protect the socket circuit.
  4. List all the tests which would have been required for this installation in the order in which they would have been earned out.
  5. Describe how to check the continuity of the radial socket circuit and how to check the polarity of the sockets.
  6. Explain how to check the polarity of the two way switches in the lighting circuit.
  7. Describe how one could confirm the breaking capacity of the new meb’s and the RCBO.
  8. List 10 things to check when inspecting the distribution board in the garage.
  9. List four things to look out for when inspecting the wiring in the garage.

 

Answers:

1. The ‘Rule of thumb’ states that a measured value of Zs may be regarded as satisfactory if it does not exceed 80 % of the maximum allowed. 

CircuitMax Zs Ω80% Max Zs ΩMeasured Zs Ω
Garage supply1.441.1520.4
Sockets2.872.2960.67
Lights7.676.2360.71

2. Carry out the insulation resistance test before the lamps are connected.

3. Additional protection by RCD is required for: 

  • Socket outlets rated at 20Aor less for general use by ordinary people.This would be the case in a domestic property.
  • Mobile equipment rated at 32A or less being used outdoors. The garage sockets may be used to supply gardening equipment used outside. People within the garage standing on a floor of concrete laid directly on top of earth are in good contact with earth.

4.

  • Continuity of protective conductors
  • Insulation resistance
  • Polarity
  • Earth fault loop impedance
  • Prospective fault current
  • Functional testing including testing of RCD’s

5. Assuming the installation to be isolated, locked off and proved dead:– 

  1. Use a low resistance ohmmeter. Check the battery. Zero out the resistance of the test leads.
  2. In the distribution board connect line and earth busbars together using a link of negligible resistance.
  3. Measure the resistance between line and earth at each socket outlet. The readings are R1 + R2 values.
  4. Record the highest value on the Schedule of Test Results.

6.    When measuring R1 + R2 on the lighting circuit operate each light switch and ensure 
that the reading goes to infinity i.e. open circuit. This will prove that the switches are not 
in the neutral. 
7.   The ultimate breaking capacity of the protective devices should be marked on them. 
The breaking capacity in Amperes is marked inside a rectangle. 
8.    

  • Correct rating of the protective devices (In and IΔn )
  • Adequate breaking capacity ( Icn) of protective devices
  • Correct characteristic type ( B, C or D ) of protective devices
  • Correct sequence of conductors on busbars
  • D.B. securely fixed to wall
  • No holes bigger than IP4X on top of D.B.
  • No holes bigger than IP2X on front sides and bottom of D.B.
  • Holes where the-twin and earth cables come out to be protected by grommets
  • SWA gland to be properly fitted
  • Earth tag on the SWA gland to be bolted to the D.B. and a lead taken from the bolt to the
  • earth busbar
  • Protective devices to be marked to show which circuit they protect
  • Notice to tell people to test the RCD quarterly
  • Terminals tight
  • No visible damage

9.

  • No excessively sharp bends
  • Cables properly clipped
  • Switch wires and pass wires sleeved brown
  • Outer sheath taken into enclosures, no cores exposed outside enclosures
  • Bare cpc to be sheathed green and yellow in enclosures
  • Insulation on live conductors should go up to terminals, no bare live conductor showing
  • No visible damage, cuts abrasion etc.